This invention relates generally to particle mixing technology and, more particularly, to an apparatus and method for mixing particles into a liquid or semi-liquid medium. In certain preferred embodiments, the invention relates to the mixing of nonmetallic reinforcing particles into molten metals or metal alloys for the production of stir-cast metal matrix composite (MMC) materials.
Metal matrix composites (MMCs), particularly those based upon aluminum alloys, have gained increasing popularity and recognition as alternative structural materials, especially for applications requiring increased stiffness, wear resistance, and strength. MMCs are usually produced by mixing nonmetallic reinforcing particles such as grit, powder, fibers or the like into a metallic matrix. For example, aluminum-based MMCs are composed typically of aluminum alloys (e.g., 6061, 2024, 7075, or A356) reinforced with ceramic particles such as silicon carbide or aluminum oxide (alumina) powder. The reinforcement provided by these particles contributes strength, stiffness, hardness, and wear resistance, in addition to other desirable properties, to the composite.
Despite their growing market, the high cost of manufacturing MMCs has hampered their ability to be priced competitively with unreinforced metallic materials. Traditionally, the fabrication of metal matrix composites has employed non-liquid methods such as the compaction of blends of ceramic particles or fibers and aluminum powders, or the metal spraying of continuous fibers in a lay-up process. Unfortunately, the high cost of metallic powders and the explosion and pyrophoric hazards associated with large quantities of powders have prevented a significant reduction in the cost of MMCs produced by this approach.
In addition, the use of liquid metals in MMC fabrication has largely been limited to the infiltration of ceramic preforms. The mixing of ceramic particles into molten aluminum using stir-cast methods has not been advantageous due to problems with the incomplete wetting of fine particles having a large surface area, as well as the rapid oxidation of a chemically reactive molten metal (e.g., aluminum) during agitation. On the other hand, the simplicity of this approach and its potential for producing low cost MMCs has led to numerous studies on the fabrication of aluminum-based MMCs through stir-casting. Numerous researchers have reported experiments involving the mixing of various ceramic powders and fibers into molten aluminum-based matrices. The equipment and methods utilized in many of these experiments were extremely simple. The equipment usually consisted of a heated crucible containing molten aluminum alloy and a motor to rotate a paddle-style impeller made of graphite or coated steel in the molten aluminum while ceramic particles were added to the surface of the molten metal (i.e., the melt). The vortex formed by the rotating impeller drew the ceramic particles into the melt and the shear developed between the impeller and the walls of the crucible helped wet the particles. The temperature was usually maintained below the liquidus temperature (in the two-phase region) to keep the aluminum alloy in a semi-solid condition, since the higher viscosity of the partially solid melt further increased the shear force created by the simple impeller. This process has been called compocasting.
The aluminum-based MMCs made by compocasting suffered from various problems. In particular, since the process was carried out under atmospheric pressure, the vortex formed by the impeller rotation drew considerable amounts of gas into the melt. Also, because the composite is sensitive to turbulence and the particles act as sites for the entrapment of gas bubbles, the solidified composites produced by compocasting were often porous. In addition, it was common for these compocast MMCs to contain numerous oxide skins due to the passing of the particles through the surface oxide into the body of the melt. Another problem with the compocasting process is the low level of shear developed by the rotating impeller in the semi-liquid matrix. Since shear is needed for wetting, the particles are generally incompletely wetted by the molten metal alloys. In sum, the quality of the composites produced by these stir-cast approaches was poor and not considered commercially viable.
The aforementioned compocasting process and other prior processing techniques used in the manufacture of metal matrix composite materials are described in detail in U.S. Pat. No. 5,531,425 to Skibo et al., the disclosure of which is incorporated herein by reference.
Today, Duralcan, a division of Alcan Aluminum Corporation, is a leader in the manufacture and sale of stir-cast aluminum-based MMCs. The technological development which led to the Duralcan process is based on an improvement in mixing efficiency combined with a reduction in gas entrapment. In this process, a low vacuum of approximately 1-5 torr is drawn over molten aluminum heated above the liquidus temperature (in the fully liquid region). The reinforcing particles are added to the surface of the melt and an impeller capable of creating a moderately high level of shear in a low viscosity melt is inserted into the molten metal and stirred at high rotational speed, as measured in revolutions per minute (rpm). The vacuum removes the air which tends to act as a buffer, cushioning the particles and preventing intimate contact with the metal. With the particles in contact with the metal from the start of the process, wetting can begin immediately. The high shear impeller physically shears the particles into the aluminum alloy, spreading the aluminum over the high surface area of the fine particles, thereby rapidly wetting them. The quality of the resulting MMC is much improved over that produced by the other techniques described above. The particles are essentially 100% wetted and there is little or no porosity in the Duralcan MMC. However, while the end product of the Duralcan process is of high quality, the high cost of manufacture, due in large part to the inefficiency of particle mixing and wetting, prevents Duralcan from fully exploiting the potential MMC market.
The Duralcan process is a batch process that can be divided into three general stages. The first stage is the incorporation of the particles into the molten aluminum, i.e., bringing the particles into intimate contact with the aluminum so that wetting can begin. This stage relies on the formation of a vortex to draw the particles into the body of the melt and a vacuum for eliminating the cushioning effect of gas at atmospheric pressure. In the second stage, the particles must be sheared into the melt through the use of a rotating impeller which produces high shear force. In general, the impeller must have sharp teeth and rotate at sufficient rotational speed in order to break up agglomerates of particles such that each particle may individually come into contact with the aluminum melt. The rotational speed requirement seems to be related to a minimum level of shear generated at a specific surface velocity of the impeller in the melt. Typically, if the rotational speed of the impeller, as measured in rpm, is too low and/or the edges of the teeth are dull, low porosity MMC material comprising well-wetted particles cannot be produced. To further enhance the level of shear, a stationary bar or baffle is positioned proximate to the perimeter of the rotating impeller. A small region of increased shear is created between the outer periphery of the impeller and the baffle. The third stage involves the slow general motion of the composite in the mixing vessel so that substantially all of the composite eventually passes through the region of high shear several times. This motion also ensures uniformity of particle distribution throughout the batch.
However, the Duralcan process, and other similar stir-cast processes practiced presently, have certain shortcomings and disadvantages. In particular, the wetting of the particles, which is the main objective of mixing, begins only when the ceramic particles that are poured on the surface of the molten metal move downward through the matrix towards the rotating impeller. This process proceeds at a slow rate because the vortex is comparatively small and the downward motion is not especially strong; also, localized shear is provided only in the proximity of the baffle. Furthermore, because the ceramic particles are added to the matrix surface, the particle feed rate must be carefully controlled so as to prevent the accumulation of particles on the surface which can, in turn, choke the agitator and further slow the mixing process. Although the impeller and baffle system is simple, rugged, and easy to repair, it is inefficient and does not take advantage of the potential region of high shear which could be made to completely surround the rotating impeller. As a result, the wetting process takes much longer than necessary because the particles must pass through the narrow shear region between the impeller and the baffle several times before the agglomerates are dispersed and the molten aluminum uniformly contacts and wets each particle.
The inefficient mixing of large quantities of MMCs also produces defects in the molten composite. More specifically, agglomerates of incompletely wetted particles may become encased in heavy stable oxide skins which form as the particles roll on the melt surface oxide before submerging and moving towards the impeller. If the oxide coating is thick, the mixing process will sometimes have insufficient intensity to break the agglomerates into individually wetted particles regardless of mixing duration. These partially wetted agglomerates persist after mixing and can lead to internal and surface defects which may be detrimental to properties such as fatigue and fracture. The aluminum oxide skins also have a detrimental effect on the MMC product, because they increase the viscosity of the composite matrix during the casting process and limit the ability to cast intricate shapes having thin walls.
Prior attempts at increasing the rate of particle wetting and decreasing the process time for particle mixing have not been wholly successful. For example, Sifferlin, in U.S. Pat. No. 3,858,640, describes the introduction of reinforcing particles into molten metal by blowing the particles into the melt using an inert gas. The large amounts of gas required to carry the particulate would immediately become entrapped in the composite matrix, which is extremely sensitive to gas and turbulence, and would result in a porous composite product. Others have described a process in which the particles are plunged under the surface of the composite matrix during mixing with a mechanical hand cylinder. This process, however, produces MMCs with numerous oxide skins since the particles are pushed down through the surface oxide into the body of the matrix.
Many of the aforementioned concerns and shortcomings relating to the prior technology for MMC stir-cast mixing also exist for particle mixing generally, especially where high shear is required to effectively mix the particles into a matrix (e.g., where the particles are not readily wetted by the matrix). Thus, there exists generally a need for an efficient apparatus and method for rapidly mixing particles into a liquid or semi-liquid matrix, particularly particles that tend to agglomerate or that are difficult to wet by the matrix, and which therefore require high shear force. More specifically, there is a continuing need for a mixing method and apparatus for producing stir-cast metal matrix composite materials which rapidly and thoroughly mix particles into a matrix comprising a molten metal, thereby reducing MMC processing costs, while avoiding common problems such as incomplete particle wetting, entrapment of gases, oxidation, and non-uniform particle distribution. The present invention fulfills these needs, and further provides related advantages.
The present invention obviates the foregoing problems and provides generally a method and apparatus for mixing particles into a liquid or semi-liquid medium and provides, more specifically, a method and apparatus for mixing nonmetallic reinforcing particles into a molten metal or metal alloy for the production of metal matrix composite (MMC) materials. The apparatus and process of this invention permit rapid mixing of particles into an MMC matrix, thereby reducing process times and, consequently, manufacturing costs. As a result, the cost of preparing MMC materials can be significantly reduced, such that MMCs can be priced competitively with unreinforced metals and metal alloys.
According to the present invention, a method and apparatus for mixing particles into a medium having a liquid or semi-liquid state are provided. In accordance with a preferred embodiment of the present invention, a method and apparatus for mixing particles into a molten metal or metal alloy for the production of stir-cast metal matrix composite materials are provided. This production process is made more efficient than prior art processes by increasing both the rate of wetting and the speed at which the particles can be added to the melt. In this process, mixing (and wetting) of the particles is improved by increasing the level of shear, as well as the size and location of the shear region. In part, the increase in shear is accomplished by increasing the rotational speed of the impeller. In addition, the shear region is positioned at the very location at which the particles are introduced into the matrix, thereby decreasing the time required for the particles to reach the shear region and significantly increasing the fraction of particles which pass through the shear region. Moreover, the particles are introduced into the matrix under the matrix surface, thereby avoiding the introduction of oxide skins into the MMC.
The present invention includes an impeller useful for mixing particles into a liquid medium contained in a vessel, which optionally may be heated or cooled. The impeller preferably comprises a hollow impeller tube having an inner passage into which particles may be directed. The particles may then be directed through the inner passage and into the body of the matrix through an open end of the impeller tube at an introduction point below the surface of the matrix. The impeller tube may further include an impeller head that preferably projects radially outward from the impeller tube. It is particularly preferred that the impeller head is positioned in close proximity to an impeller base, which preferably has contours that are generally complementary to the impeller head. It is preferred that the impeller base is similar in size and shape to the impeller head so that a region of high shear exists in the volume generally between and around the impeller base and the impeller head. The impeller head preferably has one or more teeth to provide the impact forces which aid in breaking up and dispersing particle agglomerates and to entrain a larger amount of the matrix during rotation of the impeller.
It will now be apparent from the foregoing that the method and apparatus of the present invention present a significant advance generally in the field of particle mixing and, more particularly, in the field of manufacturing metal matrix composite materials. In particular, the present invention avoids or minimizes many of the shortcomings of the prior art, while significantly decreasing the process time and cost of MMC manufacture. Other features and advantages of the present invention will become apparent from the following detailed description as well as the accompanying drawings which illustrate, by way of example, certain principles of various preferred embodiments of the invention.